The reception and demodulation of broadcast AM and FM signals are known in the art. Historically, the reception and demodulation of AM and FM signals was performed using analog circuitry in which a number of analog components received incoming signals, removed the amplitude and phase portions of the signals, and passed the information contained in the amplitude and phase portions of the signals along for further processing and output. More recently, the advent of digital circuitry has allowed designers to create radios which employ digital signal processing in the demodulation and filtering of the signals.
A digital AM/FM radio receives a broadcast signal in an analog format using an antenna. A radio frequency (RF) interface then amplifies the analog signal and passes the amplified signal to a tuner. The tuner locks onto a desired frequency component of the signal and remodulates the desired frequency component to 10.7 MHz. An IF processor then receives the remodulated analog signal, converts the analog signal to a digital representation of the signal, and then quadrature mixes down the digital signal to produce an in-phase signal and a quadrature signal. The in-phase and quadrature signals are then digitally filtered to improve the selectivity of the receiver and reduce the adjacent channel interference. A demodulation unit demodulates the in-phase signal and quadrature signal to produce a magnitude, or radius, value and a phase value. FM information and AM information are then extracted from the demodulated signals. The FM and AM information is then decimated to a lower sampling frequency, the decimation step consisting of low pass filtering and undersampling to the new sampling rate. Once the phase and magnitude signals are sampled at a lower rate, they can be further processed digitally to remove induced ignition noise and to retrieve the audio signal that was encoded at the broadcast station.
The conversion of in-phase and quadrature signals to magnitude and phase values, and the subsequent filtering of such values has remained a difficult and computationally intensive task. Most algorithms that address this conversion process require expensive hardware implementations, typically requiting fast, large, and power consuming adders and multipliers that limit the efficiency of the digital demodulation approach, thus outweighing its advantages over traditional analog means.
Several algorithms have reduced the complexity of the demodulation process, thereby increasing efficiency and reducing hardware requirements. One particular algorithm is the angle accumulation mode of the coordinate rotational digital computer (CORDIC) algorithm, an iterative procedure that approximates amplitude and phase values based upon in-phase and quadrature vectors. While the CORDIC algorithm still requires multiplications, additions, and subtractions, the multiplications are such that they can be implemented by shift registers, thus reducing the hardware requirements.
Even though the CORDIC algorithm does not require hardware multipliers, at least two shifters and three adders are needed to perform the conversion process in addition to storage elements. Thus, devices performing the CORDIC algorithm still required substantial hardware that, by itself, could not perform the digital filtering operations that were required in the subsequent decimation processes. The complexity of the prior art devices for implementing the amplitude and phase calculations and the subsequent digital filtering for the decimation, resulted in substantial cost and reduced performance, thereby limiting the overall performance of a digital demodulation approach.
Thus, there exists a need in the art for an apparatus that efficiently, and with minimal circuit components, converts in-phase signals and quadrature signals to radius values and phase values and performs subsequent filtering operations.